The present invention relates to a field effect transistor (hereafter refer to "FET"), and more particularly to the field effect transistor and the fabricating method thereof, having a channel over a triangular void formed by using characteristic of crystal growth which an effective channel length can be reduced.
As shown in FIG. 1, a conventional metal silicon field effect transistor (MESFET) is fabricated on semi-insulating substrate 10. A buffer layer 11 which may consist of p-type GaAs material is disposed on the substrate. N-type GaAs channel layer 12 is disposed on the buffer layer 11. Further, n.sup.+ -type GaAs layer 13 is sequentially formed by a conventional epitaxial growing method.
A source and a drain electrodes 14 are ohmic contacted with the n.sup.+ -type GaAs layer 13 thereon, and a gate electrode 15 is Schottky contact on the n-type GaAs layer 12 exposed by recess etching on a predetermined portion between the source and the drain electrodes.
In the conventional MESFET, when a voltage is applied to a gate electrode 15, the thickness of a depletion layer of the channel layer 12 is varied depending on the extent of the applied voltage, according to which a current between the source and the drain electrodes 14 flown through the channel layer 12 is controlled.
Such a performance of the MESFET is determined by a transconductance, and a resistance between the source and the drain electrodes. That is, the more the transconductance is and the less the resistance between the source and the drain is, the faster the operational speed becomes.
The transconductance is in inverse proportion to the length of the gate electrode 15. The resistance is reduced according as both the length of the channel and the distance between the source and the drain electrodes are short. Thus, for the purpose of attaining the high-speed performance of MESFET, it is necessary for the channel length to be short and for the resistance between the source and the drain electrodes 14 to be reduced.
Actually, great efforts have been given on the reduction of the gate length which was considered most effective to enhance the capability of the GaAs MESFET, while maintaining its basic structure, in most of the studies.
However, it has accompanied following disadvantages, namely, the gate electrodes 15 fabricated by a conventional photolithography process has a limitation in its length due to the precision in adjustment of mask. Also, there occurs a problem that the excessive reduction of the gate electrode length results in a short channel effect in the sub-micron regime.
On the other hand, in the conventional MESFET, due to a voltage difference generated between the electrode of an adjacent device and the semiconductor substrate, an undesired depletion region is formed between the channel and the substrate, resulting in that the width of the current path becomes narrow. This causes a backgating effect which inevitably changes the threshold voltage and source resistance Rs.
Of course, a buffer layer 11 is formed between the channel layer 12 and the substrate in order to prevent the backgating effect, nonetheless, the backgating effect can not be completely reduced, because of the affects of a voltage of an adjacent device or a leakage current of buffer layer owing to the increase of electric field of the channel layer 12.
It is noted that the buffer layer 11 is used to remove such a leakage current possibly occurring due to the overflow of an electron having a high energy to the substrate 10 by forming a potential barrier against the electrode as well as preventing the backgating effect. Therefore, the impurity concentration needs to be brought down to about 10.sup.14 ions/cm.sup.3 in order to make the resistance be intensified.
However, in view of process of fabricating the MESFET, since the buffer layer 11 is to form a plurality of layers within a single chamber, the impurity concentration of the buffer layer 12 is increased by the impurities doped into other layers, so that its resistance could be lowered down.
As a result, there have been many problems in the crystal growth of a qualified buffer layer having a low-concentration impurity and thousands of .ANG. in thickness.